adding pointer annotations to syscalls.master

[Brooks Davis]

I plan to commit https://reviews.freebsd.org/D14285 early next week.  It
adds SAL 2.0 annotations to syscalls in syscalls.master and code
makesyscalls.sh to strip them out in generated files.

This is based on work I started in CheriBSD and use to validate fat
pointers at the syscall boundary.  Tal Garfinkel reviewed the changes,
added annotations to COMPAT* syscalls and is using them in a record and
playback framework.  One can envision other uses such as a WITNESS-like
validator for copyin/out as speculated on in the review.

As this time we are only annotating sys/kern/syscalls.master as that is
sufficient for userspace work.  If kernel use cases materialize, we can
annotate other syscalls.master as needed.

-- Brooks